A fuel cell cause a hydrogen-containing fuel gas, which is obtained by reforming a material gas, such as a city gas, and an oxidizing gas, such as air, containing oxygen to electrochemically react with each other, thereby generating electric power and heat at the same time. A unit cell (cell) of the fuel cell includes: an electrolyte layer-electrode stack assembly constituted by a electrolyte layer and a pair of gas diffusion electrodes; gaskets; and electrically conductive separators. A groove-like gas channel through which the fuel gas or the oxidizing gas (each of which is called a reactant gas) flows is formed on a main surface of each separator which surface contacts the gas diffusion electrode. The gaskets are disposed on peripheral portions of the electrolyte layer-electrode stack assembly, and a pair of these separators sandwich the electrolyte layer-electrode stack assembly. Thus, one cell is formed. A common fuel cell is so-called a stack-type fuel cell in which the above-described cells are stacked and fastened to one another, and adjacent electrolyte layer-electrode stack assemblies are electrically connected to each other in series.
The stack-type fuel cell has manifolds through each of which the reactant gas supplied to the fuel cell is supplied to the gas channel of each cell. Each manifold is formed by connecting, one another in a stack direction of the cells, manifold holes each of which is formed on the separator and extend in a thickness direction of the separator.
The fuel cell generates heat during its operation. Therefore, in order to maintain the cell at a proper temperature, the fuel cell needs to be cooled down by, for example, cooling water. Typically, a groove-like cooling water channel through which the cooling water flows is formed per 1 to 3 cells on a main surface of the separator which is opposite to the main surface on which the gas channel is formed. Moreover, the fuel cell has a manifold through which the cooling water is supplied to the cooling water channel. Such manifold formed inside the fuel cell is referred to as an inner manifold. In the fuel cell of such inner manifold type, in light of reduction in size and cost, it is important to efficiently arrange respective component (gas channel, manifold hole, and the like) on the same surface of the separator.
However, in a case where the components are efficiently arranged, the ratio of empty spaces in the separator becomes high, and this decreases the strength of the separator. Therefore, the separator, especially the peripheral portion of each manifold of the separator, tends to be damaged. Moreover, in a case where the separator is damaged, the fuel cell cannot operate. Therefore, it is necessary to quickly detect the damage of the separator in the fuel cell stack and repair it.
Known as a fuel cell which can solve these problems is a solid polymer electrolyte fuel cell which detects the damage of the fuel cell stack by supplying an indicator gas to the fuel cell (fuel cell stack) together with the reactant gas and measuring the concentration of the indicator gas in the reactant gas discharged from the fuel cell (see Patent Document 1 for example).
Patent Document 1: Japanese Laid-Open Patent Application Publication 2001-325980